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Data Link Protocols

Data Link Protocols. Asynchronous Protocols Synchronous Protocols Character-Oriented Protocols Bit-Oriented Protocols. Data Link Protocol Categories. A protocol in data communications is the set of rules or specifications used to implement one or more layers of the OSI model.

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Data Link Protocols

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  1. Data Link Protocols • Asynchronous Protocols • Synchronous Protocols • Character-Oriented Protocols • Bit-Oriented Protocols

  2. Data Link Protocol Categories A protocol in data communications is the set of rules or specifications used to implement one or more layers of the OSI model. Data link protocol is a set of specifications used to implement the data link layer

  3. Data Link Protocol Categories Asynchronous protocols • treat each character in a bit stream independently • use start and stop bits to frame the data units • inexpensive • used primarily in modems Synchronous protocols • take the whole bit stream and chop it into characters of equal size • faster than asynchronous transmission

  4. Asynchronous Protocols

  5. XMODEM Frame - half-duplex stop-and-wait ARQ protocol Signal from receiver side - ACK - NAK - CAN (cancel) SOH –Start of Header (1 byte) CRC - Cyclic Redundancy Check

  6. YMODEM • similar to XMODEM • Major differences • Data unit = 1024 bytes • CANs = 2 • Multiple files can be sent simultaneously • ZMODEM • Combining features of both XMODEM and YMODEM • BLAST (Blocked asynchronous transmission) • More powerful than XMODEM. • Full-duplex • KERMIT • Most widely used asynchronous protocol

  7. Synchronous Protocols Character-oriented protocols (or byte-oriented protocols) - the frame or packet is interpreted as a series of characters Bit-oriented protocols - the frame or packet is interpreted as a series of bits

  8. Character-Oriented Protocols

  9. Binary Synchronous Communication (BSC) - the best-known character-oriented protocol Supports half-duplex transmission Uses stop-and-wait ARQ flow control and error correction

  10. Data Frame Simple Data Frame SYN – 0010110 STX – 0000010 ETX – 0000011 BSC Frame with Header SOH – 0000001 ITB – 0011111 Multiblock Frame

  11. Multiframe Transmission ETB – 0010111

  12. BSC Control Frames

  13. BSC Control Frames cont…

  14. Bit-Oriented Protocols IBM ISO ITU-T IEEE SDLC – Synchronous Data Link Control HDLC – High-Level Data Link Control LAPs – Link Access Protocols LAN – Local Area Nelwork

  15. HDLC • Supports half/full duplex communications over point to point or multipoint links • Characterized by: station types, configuration, response modes • Station Types: • Primary – issues commands • Secondary – issues responses • Combined – issues both command & response

  16. HDLC Configuration HDLC does not support balanced multipoint

  17. Figure 11-15 HDLC Modes Who controls the link NRM – Normal Response Mode ( unbalanced configuration) ARM – Asynchronous Response Mode ABM – Asynchronous Balanced Mode * Mode for symmetrical & balanced can be set using U-frames

  18. HDLC Frame Types Informationframe Supervisoryframe Unnumbered frame

  19. HDLC Flag F& FCS field

  20. HDLC Address Field

  21. HDLC Control Field Code (S-frame) – carry coded flow and error control information Code (U-frame) – identify the type of U-frame and its function

  22. Use of P/F Field

  23. U-Frame Control Field

  24. U-Frame Control Field Set NRM Set NRM (extended) Set ARM Set ARM (extended) Unnumbered info Disconnect Request disconnect Set initialization mode Unnumbered poll Frame reject Exchange ID Reset Request information mode

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